Stock Compensation Plans (Details 3) (USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2011 years
Summary of stock compensation awards outstanding and exercisable
Number of awards outstanding, ending balance	1,714,113
Weighted average exercise price awards outstanding, ending balance	$ 24.41
Intrinsic value awards outstanding, ending balance	$ 6,362
Weighted average remaining contractual life awards outstanding, ending balance	5.42
Number of awards exercisable, ending balance	1,054,054
Weighted average exercise price awards exercisable, ending balance	$ 23.37
Intrinsic value awards exercisable, ending balance	$ 4,495
Weighted average remaining contractual life awards exercisable, ending balance	3.48

Share based compensation arrangement by share based payment award awards exercisable intrinsic value.

No definition available.

Name:	aco_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardsExercisableIntrinsicValue
Namespace Prefix:	aco_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Share based compensation arrangement by share based payment award awards exercisable number.

No definition available.

Name:	aco_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardsExercisableNumber
Namespace Prefix:	aco_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

Share based compensation arrangement by share based payment award awards exercisable weighted average exercise price.

No definition available.

Name:	aco_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardsExercisableWeightedAverageExercisePrice
Namespace Prefix:	aco_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

Share based compensation arrangement by share based payment award awards exercisable weighted average remaining contractual term one.

No definition available.

Name:	aco_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardsExercisableWeightedAverageRemainingContractualTermOne
Namespace Prefix:	aco_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

Share based compensation arrangement by share based payment award awards outstanding intrinsic value.

No definition available.

Name:	aco_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardsOutstandingIntrinsicValue
Namespace Prefix:	aco_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Share based compensation arrangement by share based payment award awards outstanding number.

No definition available.

Name:	aco_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardsOutstandingNumber
Namespace Prefix:	aco_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

Share based compensation arrangement by share based payment award awards outstanding weighted average exercise price.

No definition available.

Name:	aco_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardsOutstandingWeightedAverageExercisePrice
Namespace Prefix:	aco_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

Share based compensation arrangement by share based payment award awards outstanding weighted average remaining contractual term one.

No definition available.

Name:	aco_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardsOutstandingWeightedAverageRemainingContractualTermOne
Namespace Prefix:	aco_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalGeneralDisclosuresAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration